A high efficiency prime mover with renewable energy storage has long been a goal of motor vehicle and stationary engine design to provide energy independence, conserve fossil fuels, and reduce emission of combustion products. While the expansion engine of the present invention is applicable to both reciprocating and rotary engines, it is especially beneficial to the gas turbine. The gas turbine offers several advantages over other engines including simplicity, reliability, low maintenance, low emissions, low weight, and ability to burn most any fuel or to run on recovered heat. It has the potential to provide a universal prime mover. It is inefficient in the motor vehicle and stationary distributed electric generation size range, however, especially with respect to variable speed operation. This is because of two factors: First, it has rotor stress limitations imposed by the pressure-speed relationship. The rotor speed is directly proportional to working fluid flow rate and compression ratio, and indirectly proportional to rotor diameter. Second, it has a high heat exchanger terminal temperature difference relative to turbine temperature drop. Both of these factors begin to adversely affect cycle efficiency at a pressure ratio less than about 3. As a result, turn-down is inefficient, exhaust temperature and rotor stresses are high with rotational speeds exceeding 100,000 rpm, and a large expensive heat exchanger is needed.
Previous efforts to adapt a gas turbine to motor vehicle use, notably the Chrysler turbine have been unsuccessful. Present efforts to employ micro-turbines for distributed electric generation are proving successful, but with marginal cost advantage. In general, problems with smaller gas turbine applications are attributable to high compression work with low density ambient intake air and exhaust gas heat recovery with large and complex regenerative heat exchangers. Several cryogenic compression engines have been built and tested to reduce compression work by, in effect, transferring compression to production and storage of liquefied air or nitrogen for compression cooling. Liquefaction work is by renewable energy or other low cost means such as off-peak electricity, therefore not chargeable to cycle efficiency. Both Brayton and Rankine cycles, either fired or with fuel-less ambient heating have been tried.
Consumption of the liquefied coolant has proved to be excessive, however, and high efficiency liquefaction is still sought after. A highly effective regenerative heat exchanger is also sought after. Most gas turbines have a heat exchanger for recovering exhaust heat to improve cycle efficiency. Large surface area and enhanced heat transfer features are combined to attain high effectiveness. Fixed area recuperators constructed of numerous tubes, brazed or welded in complex header arrangements and with enhanced heat transfer are difficult to manufacture and expensive. Another kind of heat exchanger, the rotary regenerator, attains higher effectiveness than recuperators by providing passage of the atmospheric and pressurized flow streams, alternately over the same heat transfer matrix. Seals to minimize leakage between the streams are difficult to maintain and application is limited to low compression systems.
Accordingly, objects of the prime mover of the present invention are to provide:
high cycle efficiency in a low compression prime mover of a transport vehicle drawing ambient atmospheric working fluid, while utilizing recovery of vehicle braking energy and other recoverable energy to reduce compression work of the prime mover;
high cycle efficiency throughout the speed range of a low compression prime mover of a transport vehicle, utilizing recovery of vehicle braking energy and other recoverable energy to drive the engine compressor with injected liquefied air and optionally driving a cryogenic sink for absorbing heat from and re-liquefying the working fluid to reduce compression work of the prime mover;
high cycle efficiency of a low compression prime mover for distributed electric generation, drawing ambient atmospheric working fluid, while utilizing recovery of wind, solar and other recoverable energy to reduce compression work of the prime mover;
high cycle efficiency of a low compression prime mover for distributed electric generation utilizing recovery of wind, solar and other recoverable energy to drive the engine compressor with injected liquefied air and optionally driving a cryogenic sink for absorbing heat from and re-liquefying the working fluid to reduce compression work of the prime mover;
minimal heat transfer surface area of the regenerative heat exchanger of the prime mover of the present invention;
minimal liquefied working fluid consumption of the prime mover of the present invention; and
a selection of working fluid and heat sink cryo-coolant combinations for the prime mover of the present invention.